Reactions of Platinum Terminal Polyynyl Complexes trans-(C6F5)(p-tol3P)2Pt(C≡C)nH (n = 2–4) and n-BuLi, Generation of Functional Equivalents of Pt(C≡C)nLi Species, and Derivatization with Organic and Inorganic Electrophiles

Reactions of the title complexes and n-BuLi (1.5 equiv, –45 °C) afford functional equivalents of the deprotonated species trans-(C6F5)(p-tol3P)2Pt(C≡C)nLi (n = 2–4), as assayed by subsequent additions of MeI or Me3SiCl to give trans-(C6F5)(p-tol3P)2Pt(C≡C)nMe (66–52%) or trans-(C6F5)(p-tol3P)2Pt(C≡C)nSiMe3 (63–49%). However, 31P NMR data suggest more complicated mechanistic scenarios, and small amounts of the hydride complex trans-(C6F5)(p-tol3P)2PtH (independently synthesized from the chloride complex, AgClO4, and NaBH4) are detected in most cases. Analogous sequences involving trans-(C6F5)(p-tol3P)2Pt(C≡C)2H and benzyl bromide, D2O, or W(CO)6/Me3O+ BF4– similarly afford products with Pt(C≡C)2Bn, Pt(C≡C)2D, or Pt(C≡C)2C(OCH3)=W(CO)5 linkages. The crystal structures of the tungsten and corresponding SiMe3 adduct, the three Pt(C≡C)nMe species, and hydride complex are determined.


■ INTRODUCTION
The deprotonation of terminal acetylenes to alkali metal acetylides and their subsequent functionalization with electrophiles is one of the workhorse reaction sequences of synthetic organic chemistry. 1−6 Similar deprotonations/functionalizations have been carried out with carbyne complexes of the formula L y M�C(C� C) n H, 7 which have odd numbers of sp carbon atoms.
−10 One objective has been to access adducts of very long sp carbon chains, for example with n > 25 (MC >50 M).Parallel studies of extended polyynes with organic end groups have been carried out by other researchers. 11,12All parties have sought models for the elusive polymeric sp carbon allotrope, carbyne, 13 and probed numerous physical properties as a function of chain length.Synthetic efforts have utilized various homocoupling and heterocoupling reactions of terminal alkynes.Our most recent endeavors have involved platinum-(II) end group of the formula (C 6 F 5 )(p-tol 3 P) 2 Pt. 10 As shown in Scheme 2, the platinum terminal polyynyl complexes trans-(C 6 F 5 )(p-tol 3 P) 2 Pt(C�C) n H (n = 2, PtC 4 H; 10a 3, PtC 6 H; 10a 4, PtC 8 H can be accessed by condensation of butadiyne and the corresponding platinum chloride (PtCl), followed by heterocouplings with simple trialkylsilylalkynes and protodesilylation.These complexes become progressively more demanding to isolate, a trend seen for many series of terminal polyynes, and some previously unreported alternative syntheses are detailed in the Supporting Information.Higher homologues, at least through n = 9 or (PtC 18 H), can be generated at low temperature and trapped by click chemistry. 14Still higher homologues, such as PtC 26 H, can be trapped by under special oxidative homocoupling conditions to give diplatinum complexes such as PtC 52 Pt (Scheme 2).10d In the course of optimizing routes to PtC 52 Pt and lower homologues, we sought to probe the feasibility of converting the polyynyl complexes PtC

Organometallics
efficiencies.In this paper, we report that functional equivalents of these targets are easily generated and can be derivatized with a variety of electrophiles to obtain heretofore inaccessible adducts.Crystal structures of several complexes, and some mechanistically intriguing ancillary observations, are also described.No portion of these data have been communicated.Workup of a similar preparative reaction gave a 63% yield of an air stable white solid with NMR and IR properties, as well as a microanalysis, consistent with the pentadiynyl complex trans-(C 6 F 5 )(p-tol 3 P) 2 Pt(C�C) 2 Me (PtC 4 Me; 31 P{ 1 H} NMR (CDCl 3 ) 17.2 ppm, 1 J PPt = 2672 Hz).In a one-shot experiment with t-BuLi, PtC 4 Me was isolated in 31% yield.These results were taken as evidence for the generation of a functional equivalent of PtC 4 Li, albeit with some complexities as further elaborated below.

Syntheses. As shown in
Analogous preparative sequences were carried out with isolated PtC 6 H 10a and PtC 8 H. 10c As shown in Scheme 3, PtC 6 Me and PtC 8 Me were obtained in 58−52% yields and similarly characterized.The methyl groups in PtC x Me were evidenced by 1 H NMR signals that shifted downfield with sp chain length (δ 1.62, 1.81, 1.89 ppm, respectively; 3 × s).The structures were further confirmed crystallographically, as described in a following section.However, an analogous sequence involving PtC 10 H, generated in situ under conditions where click trapping is successful, 14 did not yield detectable quantities of PtC 10 Me.A number of lithiated terminal alkynes RC�CLi have been shown to add to metal carbonyl complexes, and alkylation of the resulting adducts can afford Fischer carbene complexes. 15e have previously reported such sequences starting with the chiral rhenium complexes (η 5 -C 5 Me 5 )Re(NO)(PPh 3 )(C� C) n Li (n = 1, 2) illustrated in Scheme 1. 16 Accordingly, solutions generated from PtC The hexatriynyl and octatetraynyl complexes PtC 6 H and PtC 8 H were analogously reacted with n-BuLi, W(CO) 6 , and Me 3 O + BF 4 − .NMR analyses of samples that had always been kept at ≤0 °C showed mainly starting material.The Bro̷ nsted acidities of terminal polyynes increase with increasing numbers of triple bonds, 17 so the conjugate bases should become less basic and nucleophilic.Thus, both thermodynamic and kinetic factors may be responsible for the diminished reactivity.

Mechanistically Relevant Observations.
The preparative data in Scheme 3 convincingly establish that some functional equivalent of PtC x Li can be generated from PtC x H and n-BuLi.However, when similar lithiations of the Re(C� C) n H species in Scheme 1 or cyclopentadienyl homologues are monitored by 31 P{ 1 H} NMR at −80 °C, the PPh 3 signals shift 0−2 ppm downfield.2b,c In some cases, several closely spaced signals result, possibly reflecting different ion pairing or aggregation modes.Bruce has similarly reported that upon lithiation of (η 5 -C 5 Me 5 )Ru(dppe)(C�C) 2 H, the 31 P{ 1 H} signal shifts downfield by 2 ppm.6a For further calibration, when the cyclopentadienyl ligands of a variety of complexes of the formula (η 5 -C 5 H 5 )Re(NO)(PPh 3 )(X) are monolithiated, the 31 P{ 1 H} signals shift 3−5 ppm downfield. 18owever, in Figure 1B, most of the new signals are upf ield from that of PtC 4 H.The group of peaks at 17.7−17.1 ppm could represent unreacted PtC 4 H and/or a set of lithiated species.The sharp upfield singlet at −7.8 ppm, visually judged to be of lesser area, is close to that of the free phosphine ptol 3 P in THF (−8.2 ppm), consistent with the apparent lack of 195 Pt coupling.Surprisingly, when free p-tol 3 P was introduced prior to n-BuLi addition, or reaction mixtures were subsequently spiked with p-tol 3 P, two 31 P{ 1 H} NMR signals were always observed (e.g., − 8.1 and −7.5 ppm).At all stages, reaction mixtures were homogeneous.
The generation of several phosphorus-containing species was considered.No 31 P{ 1 H} NMR data have been reported for the phosphonium salt derived from p-tol 3 P and MeI, ptol 3 PMe + I − . 19However, the triphenyl analogue Ph 3 PMe + I − exhibits a downfield signal (21.1 ppm, CDCl 3 ) 20,21 far from the −7.8 ppm species.Triarylphosphines can be reduced to alkali metal phosphides by alkyl lithium reagents, but these chemical shifts are far upfield (e.g., Li + Ph 2 P − (C 6 D 6 ), − 22.7 ppm 22 ).Triarylphosphine oxide signals would be downfield that of PtC 4 Me. 23When the reaction in Figure 1 was monitored by 19 F{ 1 H} NMR, some new, slightly shifted signals were apparent but no major changes that might suggest a disrupted C 6 F 5 ring (Figure S2).Additional analysis is provided in Discussion Section.
During the chromatographic workups of PtC 4 Me, PtC 6 Me, PtC 8 Me, PtC 4 Bn, and PtC 4 SiMe 3 , small amounts of a common byproduct eluted after the main product.NMR data suggested that this might be the hydride complex PtH, and an isolated yield (11%) was determined for the sequence affording PtC 4 Me.An authentic sample was synthesized from PtCl, as shown in eq 1.The complex exhibited a characteristic upfield 1 H NMR signal (−6.3 ppm, CDCl 3 ) and weak IR ν PtH band (2010 cm −1 ).As depicted in Figure S1, a weak 31 P{ 1 H} NMR signal for PtH could be detected prior to the isolation of PtC 4 Me.When the reaction of PtC 4 H and n-BuLi was simply quenched with methanol, PtH was subsequently isolated in 15% yield. 24When the reaction was quenched with D 2 O, a 1 H NMR spectrum showed the PtH to be essentially completely protiated.
3. Crystallography.Single crystals of the series PtC 4 Me, PtC 6 Me, and PtC 8 Me could be grown.Their structures were solved as outlined in Table S1 and Discussion Section.
Thermal ellipsoid diagrams are given in Figure 2, all of which exhibit C 6 H 4 CH 3 /C 6 F 5 /C 6 H 4 CH 3 π stacking interactions.Key metrical parameters are summarized in Table S3.The average C 6 H 4 CH 3 /C 6 F 5 centroid/centroid distances (3.60, 3.94, 3.68 Å, respectively) quantify the visually more splayed stacking in PtC 6 Me.Additional features are analyzed in Discussion Section.
The crystal structures of the inorganic derivatives PtC 4 SiMe 3 and PtC 4 C(OMe)�W were similarly determined.The former was obtained as a solvate from toluene or in unsolvated form from CH 2 Cl 2 /hexane.Two crystals of the unsolvated form were analyzed, and the best of the three structures is depicted in Figure 3.Given the surprise associated with the initial detection of the byproduct PtH, it was crystallographically characterized prior to independent syn- thesis (eq 1).The molecular structure featured a C 2 symmetry axis, and some metrical parameters are incorporated into the caption of Figure 3.

■ DISCUSSION
Scheme 3 clearly establishes the feasibility of derivatizing PtC 4 H, PtC 6 H, and PtC 8 H with a variety of electrophiles following additions of n-BuLi.In principle, it should be possible to directly synthesize the corresponding methylation products PtC x Me by Sonogashira-type couplings of PtCl with the terminal alkynes H(C�C) n Me, analogously to the condensation with 1,3-butadiyne in Scheme 2. However, these alkynes are difficult to access, 25−27 and we judge it easier to build up the ligands in the metal coordination sphere.
Bruce has previously reported the similar functionalization of the cis bis(1,3-butadiynyl) platinum complex I in Scheme 4. 4 He found that sequential treatment with excess t-BuLi and MeI afforded the dimethylated product II in 93% yield after workup.However, analogous sequences with Me 3 SiCl and AuCl(PPh 3 ) afforded monosilyl and monoaurated derivatives that retained one (C�C) 2 H ligand. 31 P{ 1 H} NMR spectra were not recorded during these transformations, but like our experience with Figure 1, might have proved difficult to interpret.
Individually, the crystal structures of PtC 4 Me, PtC 6 Me, and PtC 8 Me are routine, with bond lengths, bond angles, and arene/arene stacking interactions similar to many other alkynyl and polyynyl adducts of trans-(C 6 F 5 )(p-tol 3 P) 2 Pt. 10,14In accordance with computational predictions 28 and experimental analyses, 29 the Pt−C� bond lengths contract from 2.009(2) to 1.983(3) Å.In principle, the carbon−carbon bond lengths should also exhibit monotonic trends, but as is often the case with lighter atoms, the ESD values are too high for rigorous conclusions.In PtC 4 C(OMe)�W, the bond lengths involving the atoms between platinum and tungsten are potentially influenced by the zwitterionic resonance form + M(�C� C) n �C(OMe)-W(CO) 5 − .However, the Pt−C� bond length (2.003(5) Å) is close to that of PtC 4 Me.In contrast, the W� C bond (2.147(5) Å) is slightly shorter than in (η 5 -C 5 Me 5 )Re(NO)(PPh 3 )C�CC(OMe)�W(CO) 5 (2.200(8) Å), 16 which is anchored by a π basic rhenium fragment that should enhance zwitterionic character and reduce the tungsten−carbon bond order.
As noted above, 31 P{ 1 H} NMR data for the family of rhenium complexes in Scheme 1 2 and related ruthenium complexes of Bruce et al. 6a suggest that ancillary phosphine ligands in MC 4 Li species should have chemical shifts similar to those of MC 4 H analogues, or a few ppm downfield.No such downfield signals are apparent in Figure 1.However, since the literature data are derived from 18-valence-electon octahedral complexes, it may not be unreasonable that resonances associated with 16-valence-electron square planar PtC 4 Li are among the slightly upfield group of signals at 17.7−17.1 ppm.An "ate complex" derived from n-BuLi addition to platinum has been considered, but this has little precedent. 30ucleophilic aromatic substitutions involving fluoroarenes have abundant precedent, but species derived from addition   31 and our 19 F{ 1 H} spectra (Figure s2) show only sp 2 CF signals. 32he formation of the byproduct PtH in so many of the preceding reactions also poses a puzzle.However, in syntheses of extended polyynes by certain types of oxidative cross-and homocoupling reactions, the loss of C 2 units is sometimes observed. 11,14,33There are currently no rationales for these well-documented minor reaction pathways, which are quite possibly related.In the same vein, we presently have no explanation for the generation and then disappearance of the −7.8 ppm 31 P{ 1 H} NMR signal in Figure 1 (which seems not to be p-tol 3 P).
In an effort to further extend this chemistry, the reaction mixtures generated from PtC x H and n-BuLi were treated with various one-electron oxidants.It was hoped that homocouplings to diplatinum complexes PtC 2x Pt might be effected.However, complex product mixtures were produced.Nonetheless, transmetalation chemistry remains worthy of exploration.For example, the rhenium analogues (η 5 -C 5 Me 5 )Re-(NO)(PPh 3 )(C�C) n Li undergo Li/Cu exchange to give species that efficiently condense with brominated alkynes and diynes. 8Thus, the deprotonation products of PtC x H continue to have considerable promise for sp chain elongation protocols.
In conclusion, this study has extended a class of reactions that we developed for octahedral rhenium terminal polyynyl complexes in 1991 (Scheme 1) to square planar platinum terminal polyynyl complexes trans-(C 6 F 5 )(p-tol 3 P) 2 Pt(C� C) n H (n = 2−4; Scheme 3).−6 These can be derivatized by a variety of electrophiles and have promise for various heterocoupling and oxidative homocoupling reactions.Despite the occasional mechanistic puzzle, such sequences have much potential for synthetic organometallic chemistry and continue to receive attention in this research group.

■ EXPERIMENTAL SECTION
All instrumentation and characterization protocols were identical to those in recent full papers in this series.10c,10,14 These are summarized, together with chemical sourcing and purification, in the Supporting Information.All reactions were conducted under dry inert atmospheres using conventional Schlenk techniques, but workups were carried out in air.
Crystallography.The following structure solution is representative, and others are detailed in the SI.A CH 2 Cl 2 solution of PtC 4 Me was layered with hexanes and kept at 4 °C.After 3 days, colorless blocks were collected.Cell parameters were determined from 60 data frames taken at widths of 0.5°and refined with 111,645 reflections using CrysAlisPro. 41Numerical absorption corrections were based on Gaussian integrations over a multifaceted crystal model.Empirical absorption corrections were performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.Systematic reflection conditions and statistical tests suggested the space group P2 1 /n, which was confirmed by SHELXT. 42Hydrogen atom positions were calculated and refined using a riding model.All non-hydrogen atoms were refined anisotropically.The absence of additional symmetry and voids was confirmed using PLATON (ADDSYM). 43he structure was refined (full matrix least-squares refinement on F 2 ) to convergence. 43,44ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.4c00098.Experimental section; tables; variable-temperature NMR spectra; mass spectrometric analysis of PtC 4 D; and NMR spectra of new complexes (PDF) Accession Codes CCDC 2311910−2311913, 2332973−2332974, 2334405, and 2337165 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge via Organometallics Scheme 2. Background Platinum Chemistry; Sources of Key Starting Materials

1 J
Scheme 3, a THF solution of PtC 4 H was treated with n-BuLi (1.5 equiv, 2.5 M in hexane) at −45 °C.The yellow solution turned orange.As depicted in Figure 1, the 31 P{ 1 H} NMR signal of PtC 4 H (18.3 ppm, 1 J PPt = 2644 Hz) was replaced by (i) a weaker, slightly shifted singlet overlapping with a less intense group of broad signals (17.7− 17.1 ppm) immediately upfield, (ii) a sharp upfield signal (−7.8 ppm) noteworthy for the absence of 195 Pt satellites, and (iii) several minor signals.The solution was then warmed to 0 °C and MeI (1.8 equiv) added.The 31 P{ 1 H} NMR spectrum was now dominated by a new 195 Pt-coupled signal (16.5 ppm, PPt = 2684 Hz).
Next, solutions generated from PtC x H and n-BuLi at −45 °C were quenched (0 °C) with the silicon electrophile Me 3 SiCl (1.8 equiv).As shown in Scheme 3, trans-(C 6 F 5 )(ptol 3 P) 2 Pt(C�C) n SiMe 3 (n = 2, PtC 4 SiMe 3 ; 3, PtC 6 SiMe 3 ; 10c 4, PtC 8 SiMe 3 10c ) were isolated as air stable white to yellow solids in 63−48% yields.The last two complexes have been independently prepared by sequences similar to those in Scheme 2, but the first represents a "missing link".Solutions generated from the butadiynyl complex PtC 4 H and n-BuLi were similarly treated with other types of electrophiles.In the case of benzyl bromide, workup gave the new compound trans-(C 6 F 5 )(p-tol 3 P) 2 Pt(C�C) 2 CH 2 Ph (PtC 4 Bn) in 47% yield.When solutions were quenched with D 2 O (99.9%), PtC 4 D was isolated in 48% yield.Integration of the residual C 4 H 1 H NMR signal versus the aryl hydrogen atoms and mass spectrometric analyses indicated 85−82% deuterium labeling.